Neisseria gonorrhoeae (gc), the etiologic agent of gonorrhea, was grown in liqud and on solid bacteriologic media, in the allantoic fluid of 10-day chicken embryos and in transudate (serum-like fluid) within small chambers (5 ml.) implanted subcutaneously in the backs of guinea pigs. The bacteria were incubated with increasing concentrations of PBS dialyzed, acetate extracts of granules purified from human neutrophilic granulocytes. Experiments were done to compare the sensitivity of: a) virulent (colony type 1) and avirulent (colony type 4) gc, and b) gc grown by various methods, to the bactericidal components found within human neutrophil granules (lysosomes). Two isolates of gc were used; F62, a widely used strain which has been passaged in many laboratories for almost 20 years, and GC7, a recently isolated strain obtained from cervical discharge and passaged since mid-1978 in this laboratory. Virulent and avirulent gc of both strains showed very similar responses to killing by granule extract. Sensitivity to 250 microgram/ml of granule extract decreased in the order: log-phase egg-grown, log-phase broth grown, stationary-phase egg-grown, guinea pig chambergrown and stationary-phase agar-grown. Dose response curves (10-500 microgram/ml) for egg-grown and broth-grown gc were sigmoidal, whereas those for guinea-pig and agar-grown gc were convex curves. Only log-phase egg-grown gc were killed 10% by 300-500 microgram/ml of granule extract. Heating the granule extract at 10 degrees C for 20 min abrogated its bactericidal effect. Bactericidal activity increased over time and was temperature dependent. Myeloperoxidase/Cl-/H2O2 killed type 1 and type 4 gc to the same degree.